1. Field of the Invention
The present invention relates to a single crystal growth method, and more particularly to a method for growing single crystals for use in an optical isolator, high temperature superconducting cable and the like.
2. Description of the Related Art
Magnetic garnet single crystals containing iron, exemplified by yttrium--iron--garnet single crystal (Y.sub.3 Fe.sub.5 O.sub.12 : hereinafter abbreviated as "YIG"), having a large Faraday effect have been used as a material for optical isolators.
Further, oxide superconductors, exemplified by yttrium-barium-copper oxide (YBa.sub.2 Cu.sub.3 O.sub.x : hereinafter abbreviated as "YBCO"), for which low cost liquid nitrogen may be used as a coolant have been typically used as a material for superconducting cables.
These compounds are known as incongruent melting compounds and single crystals cannot be obtained directly from a raw material having a congruent melting composition. Therefore, method for growing single crystal by using a flux having a special composition has generally been employed.
More specifically, as shown in FIG. 1, YIG is a incongruent melting compound and is decomposed at about 1585.degree. C. when its temperature is increased. However, YIG cannot be obtained even if a melt having a stoichiometric composition is coagulated and is decomposed into ortho-ferrite (YFeO.sub.3) and liquid phase. Therefore, a method for growing YIG single crystal by means of a flux method or an LPE (liquid phase epitaxy) method has been adopted in which a solvent is prepared in which lead oxide (PbO) and diboron trioxide (B.sub.2 O.sub.3) are mixed within a crucible made of Pt and by using a solution in which diiron trioxide (Fe.sub.2 O.sub.3) and diyttrium trioxide (Y.sub.2 O.sub.3) are dissolved as solutes. The flux method allows bulk single crystals to be obtained by putting seed crystals in the above-mentioned solution and by slowly cooling it. The LPE method allows a thin film single crystal to be obtained by growing crystals on a gadolinium--gallium--garnet (Gd.sub.3 Ga.sub.5 O.sub.12 : hereinafter abbreviated as "GGG") single crystal substrate.
Further, there is known a floating zone melting method (hereinafter referred to as a FZ method) for growing single crystals. The FZ method involves growing single crystals by maintaining a polycrystal raw material within a heating furnace, creating a melt zone by heating only a certain portion to a temperature higher than the melting point and moving the melt zone. The FZ method has merits that it allows one to avoid impurities from being introduced from a crucible member because it uses no crucible, that the atmosphere can be arbitrarily set and that it allows single crystals of a high melting point substance to be fabricated.
There has been also known a traveling solvent floating zone method (hereinafter abbreviated as a TSFZ method) as one species of the FZ method. The TSFZ method of growing single crystal involves providing a seed crystal solvent whose composition and weight have been precisely controlled, heating and fusing to be fully congruent and then by combining with a polycrystal as a raw material (S. Kimura et.al., J. Cryst. Growth, 41 (1977)192-198). Because this method allows bulk single crystals of the incongruent melting compound to be grown from a melt, single crystals of oxide high temperature superconductor are now actively grown by using the TSFZ method.
The inventors have grown fibrous YIG single crystal by the FZ method by using a YAG laser heating image furnace mounting a YAG laser as an optical heating unit (Collection of Manuscripts of Lecture, the 42nd Spring Lecture Meeting of Association Related to Applied Physics, No. 1, 28p-TA-16, 1995).
As a result of intensive research, the inventors have found that YIG grows directly from the surface of the seed crystal without using solvent intentionally when the specimen is fiberized. Studying this mechanism of growth, the inventors have found that it is a self-adjusting reaction in that YFeO.sub.3 (ortho-ferrite), the initial phase, grows at the melt zone part right under the joint between the polycrystal raw material and the seed crystal and that YIG deposits as the remaining liquid phase rich in Fe becomes a fusing agent (Sekijima, et. al.: The 27th Domestic Conference On Crystal Growth, 801a1B3, 1996).
It is noted that the single crystal growth apparatus which the inventors have used is designed so as to be able to create a very small melt zone as compared to a normal optical heating unit in order to deal with the fibrous and thin specimen. Therefore, the above-mentioned self-adjusting reaction occurs instantaneously at the very small area.
Because ortho-ferrite deposits as the initial phase at the joint between the raw material and the seed crystal in the conventional FZ method, the growth orientation of the single crystal could not be controlled. As a result, the single crystal had to be grown by the TSFZ method to control the growth orientation of the single crystal. However, it is physically difficult to position solvent on the seed crystal in obtaining thin crystal whose diameter is less than 3 mm. Still further, because the amount of solvent is very small, it is also difficult to control its weight. For these reasons, there has been a problem that when the amount of solvent is inadequate, the shape of the obtained single crystal becomes unstable because the composition fluctuates during growing the single crystal.
There has been also the trouble in the TSFZ method that the shape of the melt zone becomes unstable and the melt zone is cut off from growing the single crystal as the melt within the melt zone infiltrates into the polycrystal raw material.
For the forgoing reasons, there is a need for a single crystal growth method which allows the single crystal of the incongruent melting compound to be stably grown while controlling the growth orientation.